A standard centrifuge for separating a slurry into solid and liquid fractions has a foraminous drum that is rotated at high speed about its axis. The slurry is fed into the drum so that the liquid fraction is driven centrifugally out, leaving a cake of the solid fraction adhering to the inner surface of the drum. The operation typically takes place in a wholly closed chamber so that a pressure differential can be applied to maximize the separation effect and to aspirate as much as possible of the liquid phase.
It is standard to strip out the solid fraction by means of a blade which is positioned inside the drum to skim off the inner surface of the normally cylindrically tubular mass of the solid-phase cake resting on the inner face of the separation drum. This blade normally has an outer edge extending parallel to the axis and is pointed against the rotation direction of the drum.
In order to control the stripping operation the blade is mounted as described in U.S. Pat. No. 3,779,450 so that it can be radially displaced in the drum. To this end the blade is carried on the outer end of generally radially extending arms whose inner ends are fixed on a shaft extending parallel to but offset from the drum axis and projecting out of the housing through a seal arrangement. Pivoting of this shaft about its own axis therefore angularly displaces the blade edge arcuately so that some control over the radial spacing of the blade edge from the inside housing surface is obtained. Unfortunately such control is fairly crude, as the movement arc of the blade is normally set to just tangent the inner drum surface. As a result there is a compound trigonometric relationship between the angular position of the blade shaft and the radial spacing between the blade and the drum.
In another known system described in German patent document 1,432,886 filed 03 March 1984 by H. J. Titus a basically pivotal blade can be swept parallel to the drum axis along the drum inner surface to strip off the filter cake thereon. This arrangement still has the essentially pivotal system that makes accurately positioning the blade very difficult. In this arrangement, once again, the continuously changing angle of attack of the blade as it is adjusted makes the stripping force different and generally causes the parameters of the stripping action to change.